The present invention is directed toward a wavelength division multipelxed (WDM) ring transmission system.
Wavelength division multiplexing (WDM) is being explored as an approach for economically increasing the capacity of existing fiber optic networks. WDM systems typically include a plurality of transmitters, each respectively transmitting signals on a designated wavelength or channel. As a result, fiber capacity can be increased by a multiple equal to the number of channels.
WDM systems have been deployed in long distance networks in a point-to-point configuration consisting of end terminals spaced from each other by one or more segments of optical fiber. In metropolitan areas, however, WDM systems having a ring or loop configuration are currently being developed. Such systems typically include a plurality of nodes located along the ring. At least one optical add/drop multiplexer, associated with each node, is typically connected to the ring. The optical add/drop element permits both addition and extraction of channels to and from the ring. One of the nodes, referred to as a hub or central office node, typically has a plurality of associated add/drop elements for transmitting and receiving a corresponding plurality of channels to/from other nodes along the ring.
Each optical signal in a WDM system is typically transmitted at a unique wavelength. Conventional filters, such as dielectric filters, can be provided within the add/drop elements in order to facilitate adding and/or dropping of individual channels, while allowing the remaining optical signals to continue along the ring. Each filter and its associated optical connectors, however, imposes an incremental loss on optical signals propagating along the WDM ring. Accordingly, a limited number of optical add/drop elements are typically coupled to the WDM ring in order to minimize loss. Moreover, the cost of each optical add/drop element can be relatively high. Thus, the number of optical add/drop elements coupled to the WDM ring is restricted for economic reasons as well.
Currently, single wavelength time division multiplexed (TDM) rings have been deployed in many metropolitan areas. In these systems, TDM add/drop multiplexers (e.g., synchronous optical network or SONET add/drop multiplexers) assign a particular time slot to each signal source, so that a single complete signal is constructed from portions of the signal collected from each time slot. While this is a useful technique for carrying plural information sources on a single channel, it is prohibitively expensive to continue to increase the TDM data rate. Thus, numerous fiber rings have been deployed, each respectively coupled to a group of TDM add/drop multiplexers.
In order to increase the bandwidth or capacity of such TDM networks, it would be advantageous to couple a plurality of TDM add/drop multiplexers to a single fiber ring by assigning each TDM add/drop multiplexer a particular WDM wavelength which, in turn, could be added and dropped from the ring by a corresponding optical add/drop multiplexer. As noted above, however, the loss and expense associated with each optical add/drop multiplexer limits the number optical add/drop multiplexers. Accordingly, the number of TDM add/drop multiplexers which can be coupled to a WDM ring with optical add/drop multiplexers is similarly restricted.